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Study On Flexible DC Grid Based On PWM Current Source Converter

Posted on:2020-07-09Degree:MasterType:Thesis
Country:ChinaCandidate:X D YinFull Text:PDF
GTID:2392330572483811Subject:Power system and its automation
Abstract/Summary:PDF Full Text Request
With the development and utilization of renewable energy,the advantages of HVDC transmission have gradually emerged.Among them,flexible DC transmission technology overcomes the shortcomings of traditional HVDC transmission.It has many advantages,such as no commutation failure,flexible control mode,no reactive power compensation equipment,and easy to build multi-terminal HVDC power grid.It is being widely studied by scholars at home and abroad.However,if the DC side short circuit fault occurs,even if the switch in two-level,three-level VSC or half-bridge sub-module MMC is blocked,the fault current will still flow to the fault point through the reverse parallel diode in the converter valve,thus the fault clearance can not be achieved through the control of the converter itself.Therefore,this paper proposes a current source PWM converter(PWM-CSC),which is dual to the voltage source converter structure,to analyze the topology of traditional PWM-CSC and improved PWM-CSC,to study the modulation strategy of PWM-CSC and the design of controller for AC network balance,and to further study the soft start-up control strategy,the power flow control strategy,the DC fault treatment and recovery strategy based on PWM-CSC flexible DC grid.By comparing the switching states of PWM-CSC and VSC,this paper establishes the mapping relationship between PWM-CSC switching signals and corresponding VSC switching signals by using the mathematical relationship between two-logic signals and three-logic signals,and establishes the modulation strategy based on PWM-CSC.Using current vector control,various types of power controllers are designed to realize PWM-CSC power control when AC power network is balanced.At the same time,a three-terminal flexible DC power grid is established based on PWM-CSC.The generation mechanism of over-voltage and over-current during the start-up of the flexible DC power grid based on PWM-CSC is analyzed.Referring to the soft-start strategies of the existing flexible DC power grid and various hybrid power grids,the soft-start strategy of the flexible DC power grid based on PWM-CSC and the grid-connected method of the new converter station are proposed.In addition,in order to realize the power flow control of flexible DC power grid,the basic principles of margin control,droop control and TPFC are studied,and their application in flexible DC power grid based on PWM-CSC is also studied.Then,in view of DC fault such as short circuit of DC line,the co-operation between converter and DC circuit breaker can be realized by changing the control parameters of power controller of converter,and the cost of DC circuit breaker can be reduced at the same time.Finally,the simulation model of the above control strategy is built in PSCAD/EMTDC and verified by simulation.The simulation results show that the various modulation strategies,control strategies,DC fault handling and recovery strategies described in this paper can achieve the desired effect,and the method is feasible.
Keywords/Search Tags:flexible DC transmission, current source converter, soft start-up control strategy, power flow control strategy, DC fault
PDF Full Text Request
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